Tomoo Katsura

Olivine‐wadsleyite transition in the system (Mg,Fe)2SiO4

Phase relations of the olivine-wadsleyite transition in the system (Mg,Fe) 2 SiO 4 have been determined at 1600 and 1900 K using the quench method in a Kawai-type high-pressure apparatus. Pressure was determined at a precision better than 0.2 GPa using in situ X-ray diffraction with MgO as a pressure standard. The transition pressures of the end-member Mg 2 SiO 4 are estimated to be 14.2 and 15.4 GPa at 1600 and 1900 K, respectively. Partition coefficients for Fe and Mg between olivine and wadsleyite are 0.51 at 1600 K and 0.61 at 1900 K. By comparing the depth of the discontinuity with the transition pressure, the temperature at 410 km depth is estimated to be 1760 ± 45 K for a pyrolitic upper mantle. The mantle potential temperature is estimated to be in the range 1550-1650 K. The temperature at the bottom of the upper mantle is estimated to be 1880 ± 50 K. The thickness of the olivine-wadsleyite transition in a pyrolitic mantle is determined to be between 7 and 13 km for a pyrolitic mantle, depending on the efficiency of vertical heat transfer. Regions of rapid vertical flow (e.g., convection limbs), in which thermal diffusion is negligible, should have a larger transition interval than stagnant regions, where thermal diffusion is effective. This is in apparent contradiction to short-period seismic wave observations that indicate a maximum thickness of <5 km. An upper mantle in the region of the 410 km discontinuity with about 40% olivine and an Mg# of at least 89 can possibly explain both the transition thickness and velocity perturbation at the 410 km discontinuity.

Mineralogy of subducted basaltic crust (MORB) from 25 to 37 GPa, and chemical heterogeneity of the lower mantle

Abstract Experimental multi-anvil study of the phase relations in subducted oceanic crust (mid-ocean ridge basalt (MORB)) composition over a pressure range from 25 to 37 GPa, equivalent to depths of 700–950 km in the lower mantle, reveals that at pressures between 25 and 30 GPa majoritic garnet disproportionates into the Mg- and Ca-perovskites and an aluminous calcium ferrite type phase, but no further change in mineralogy takes place at higher pressures up to 37 GPa. The intermittent seismic discontinuity, at least that at 920 km depth reported by Kawakatsu and Niu [H. Kawakatsu, F. Niu, Nature 371 (1994) 301–305], might be attributable to these mineralogical changes in the former basaltic crust of subducted slabs. Moreover, we estimate that a density profile of MORB intersects an average mantle density at around 1500–2000 km depth in the lower mantle. The neutral buoyancy may contribute to the observed transition in seismological heterogeneity at a depth around 1600 km.

Hydrous olivine unable to account for conductivity anomaly at the top of the asthenosphere

The oceanic asthenosphere is observed to have high electrical conductivity, which is highly anisotropic in some locations. In the directions parallel and normal to the plate motion, the conductivity is of the order of 10-1 and 10-2 S m-1, respectively, which cannot be explained by the conductivity of anhydrous olivine. But because hydrogen can be incorporated in olivine at mantle pressures, this observation has been attributed to olivine hydration, which might cause anisotropically high conductivity by proton migration. To examine this hypothesis, here we report the effect of water on electrical conductivity and its anisotropy for hydrogen-doped and undoped olivine at 500–1,500 K and 3 GPa. The hydrous olivine has much higher conductivity and lower activation energy than anhydrous olivine in the investigated temperature range. Nevertheless, extrapolation of the experimental results suggests that conductivity of hydrous olivine at the top of the asthenosphere should be nearly isotropic and only of the order of 10-2 S m-1. Our data indicate that the hydration of olivine cannot account for the geophysical observations, which instead may be explained by the presence of partial melt elongated in the direction of plate motion.

Core formation in planetesimals triggered by permeable flow

The tungsten isotope composition of meteorites indicates that core formation in planetesimals occurred within a few million years of Solar System formation. But core formation requires a mechanism for segregating metal, and the ‘wetting’ properties of molten iron alloy in an olivine-rich matrix is thought to preclude segregation by permeable flow unless the silicate itself is partially molten. Excess liquid metal over a percolation threshold, however, can potentially create permeability in a solid matrix, thereby permitting segregation. Here we report the percolation threshold for molten iron–sulphur compounds of approximately 5 vol.% in solid olivine, based on electrical conductivity measurements made in situ at high pressure and temperature. We conclude that heating within planetesimals by decay of short-lived radionuclides can increase temperature sufficiently above the iron–sulphur melting point (∼1,000 °C) to trigger segregation of iron alloy by permeable flow within the short timeframe indicated by tungsten isotopes. We infer that planetesimals with radii greater than about 30 km and larger planetary embryos are expected to have formed cores very early, and these objects would have contained much of the mass in the terrestrial region of the protoplanetary nebula. The Earth and other terrestrial planets are likely therefore to have formed by accretion of previously differentiated planetesimals, and Earths core may accordingly be viewed as a blended composite of pre-formed cores.

Dry mantle transition zone inferred from the conductivity of wadsleyite and ringwoodite

The Earth’s mantle transition zone could potentially store a large amount of water, as the minerals wadsleyite and ringwoodite incorporate a significant amount of water in their crystal structure. The water content in the transition zone can be estimated from the electrical conductivities of hydrous wadsleyite and ringwoodite, although such estimates depend on accurate knowledge of the two conduction mechanisms in these minerals (small polaron and proton conductions), which early studies have failed to distinguish between. Here we report the electrical conductivity of these two minerals obtained by high-pressure multi-anvil experiments. We found that the small polaron conductions of these minerals are substantially lower than previously estimated. The contributions of proton conduction are small at temperatures corresponding to the mantle transition zone and the conductivity of wadsleyite is considerably lower than that of ringwoodite for both mechanisms. The dry model mantle shows considerable conductivity jumps associated with the olivine–wadsleyite, wadsleyite–ringwoodite and post-spinel transitions. Such a dry model explains well the currently available conductivity–depth profiles obtained from geoelectromagnetic studies. We therefore conclude that there is no need to introduce a significant amount of water in the mantle transition to satisfy electrical conductivity constraints.

Sound velocities and elastic properties of Fe‐bearing wadsleyite and ringwoodite

The sound velocities and single-crystal elastic moduli of β phase (wadsleyite) and γ phase (ringwoodite) of (Mg,Fe)2SiO4 with Fe/(Fe+Mg) ratios of ∼0.075 and ∼0.09, respectively, have been determined at ambient conditions by Brillouin spectroscopy. Both compressional and shear wave aggregate velocities decrease with increasing Fe content in both phases, but the magnitude of this decrease is different for the two phases. The adiabatic bulk modulus, Ks, of Fe;-bearing β-Mg2SiO4 (Ks = 170±2 GPa) is indistinguishable from that of the Mg end-member within experimental uncertainty, whereas Ks of γ-(Mg,Fe)2SiO4 increases rapidly with increasing iron content. The shear moduli of both phases decrease with increasing Fe content. Our measurements indicate that the velocity and impedance contrasts between olivine and β-(Mg,Fe)2SiO4 are independent of Fe content for Mg-rich compositions, but the contrast for the β → γ-(Mg,Fe)2SiO4 transition increases significantly with increasing Fe content. The new data support a previous estimate of 40±10% for the olivine content of the upper mantle and suggest that less than 50% (Mg,Fe)2SiO4 is sufficient to account for the observed impedance contrasts at depths of both 410 km and 520 km. Unless the effect of Fe on elastic properties is accounted for, it is difficult to account for both the 410 and 520 km discontinuities with a single olivine content.

Small effect of water on upper-mantle rheology based on silicon self-diffusion coefficients

Water has been thought to affect the dynamical processes in the Earth’s interior to a great extent. In particular, experimental deformation results suggest that even only a few tens of parts per million of water by weight enhances the creep rates in olivine by orders of magnitude. However, those deformation studies have limitations, such as considering only a limited range of water concentrations and very high stresses, which might affect the results. Rock deformation can also be understood as an effect of silicon self-diffusion, because the creep rates of minerals at temperatures as high as those in the Earth’s interior are limited by self-diffusion of the slowest species. Here we experimentally determine the silicon self-diffusion coefficient DSi in forsterite at 8 GPa and 1,600 K to 1,800 K as a function of water content CH2O from less than 1 to about 800 parts per million of water by weight, yielding the relationship, DSi ≈ (CH2O)1/3. This exponent is strikingly lower than that obtained by deformation experiments (1.2; ref. 7). The high nominal creep rates in the deformation studies under wet conditions may be caused by excess grain boundary water. We conclude that the effect of water on upper-mantle rheology is very small. Hence, the smooth motion of the Earth’s tectonic plates cannot be caused by mineral hydration in the asthenosphere. Also, water cannot cause the viscosity minimum zone in the upper mantle. And finally, the dominant mechanism responsible for hotspot immobility cannot be water content differences between their source and surrounding regions.

Electrical conductivity of silicate perovskite at lower-mantle conditions

Geophysical models of the electrical conductivity of the Earths mantle based on the observed variations of electric and magnetic fields at the surface of the Earth yield estimates of about 1 S m−1 for the conductivity of the uppermost lower mantle,. But laboratory conductivity measurements on silicate perovskite (thought to be the dominant constituent of the lower mantle) at high pressures have given conflicting estimates of mantle conductivity, ranging from less than 10−5 up to 1 S m−1 (refs 3–6). Here we present measurements of the electrical conductivity of perovskite in a multi-anvil press at conditions appropriate for the uppermost lower mantle (pressures up to 23 GPa and temperatures up to 2,000 K). We find that the geophysical estimate of lower-mantle electrical conductivity can be well explained by the conductivity of the perovskite component of a low-oxygen-fugacity mantle composed of pyrolite (the assemblage of mineral phases thought to broadly represent that of the Earths mantle), assuming a standard geotherm. Our results also indicate that the temperature dependence of perovskite conductivity at lower-mantle temperatures and pressures is significantly larger than shown previously; extrapolations of low-temperature conductivity measurements to the higher temperatures of the lower mantle should therefore be treated with caution.

Melting and subsolidus phase relations in the MgSiO3MgCO3 system at high pressures: implications to evolution of the Earth's atmosphere

Abstract Melting and subsolidus phase relations in the MgSiO3 MgCO3 system were investigated at 8 and 15 GPa and temperatures up to 2100°C and at 26 GPa and 1600°C using a uniaxial split-sphere anvil apparatus. Mineral assemblages of enstatite + magnesite and perovskite + magnesite are stable at 8 and 15 GPa and at 26 GPa, respectively, under subsolidus conditions. The melting temperature of magnesite increases with increasing pressure. Melting of the system shows a simple eutectic relation; the eutectic point shifts to lower MgCO3 content and to higher temperature with increasing pressure. Carbon dioxide is no longer a volcatile component at a depth of 140–720 km in the mantle and is retained as magnesite. A considerable amount of CO2 has been stored in the Earths mantle for a long period of the Earths history and is successively released through volcanism.

In situ Observation of ilmenite‐perovskite phase transition in MgSiO3 using synchrotron radiation

In situ observations of the ilmenite-perovskite transition in MgSiO3 were carried out in a multianvil high-pressure apparatus interfaced with synchrotron radiation. The phase boundary between ilmenite and perovskite in the temperature range of 1300–1600 K was determined to be P (GPa) = 28.4(±0.4) - 0.0029(± 0.0020)T (K) based on Jamiesons equation of state of gold [Jamieson et al., 1982] and P (GPa) = 27.3(±0.4) - 0.0035(±0.0024)T (K) based on Andersons equation of state of gold [Anderson et al., 1989]. The consistency of our results, using Jamiesons equation of state, with previous studies obtained by quench methods leads us to conclude that the 660 km seismic discontinuity in the mantle can be attributed a phase transition to perovskite phase. However, the phase boundary based on the Andersons equation of state implies that the depth of the 660-km seismic discontinuity does not match the pressure of this transition.